Expanding on hybrid2d4x4's post, I tried replacing the stock fans on my ACX cooler. The stock fans are integrated into the shroud, so a direct replacement would be difficult. I opted to remove the shroud entirely and strap on two standard 92mm fans. I used Arctic Cooling PWM case fans (92mm x 92mm x 25mm) because their cabling allows PWM sharing without a splitter cable. One of the fans fit perfectly, the other was about 1mm too wide for its slot on the heatsink so required a small shave.
 
The results are both promising and frustrating. These fans have fluid dynamic bearings instead of double ball bearings so are noticeably quieter than the stock fans at any given RPM. That's the key improvement because they're spinning at the same minimum 1200RPM as the stock fans. Their nominal range is 500-2000RPM, but even running at what the BIOS claims to be 43%, they're spinning at 1200RPM. Next I'll try flashing the new BIOS to see if they'll spin more slowly at 31%. In any case, the card is now operating at a similar noise level to other components, so I'm not too concerned about slowing the fans further. Temperatures have not noticeably changed at any load.
 
For anyone considering doing the same, I'll just note a couple things.
1. The fan header on VGAs is somewhat smaller than standard, so you'll want to either solder/twist some cables together or get an adapter (I used a Gelid 4pin-to-4pin cable, model CA-PWM-02).
2. Try to find a fan that can handle high static pressure. If it's a model that's often paired with a heat sink, that's a good sign.
3. If the fans will be mounted to blow vertically, as they would in a standard tower case, sleeve bearings are a bad idea. They won't lubricate correctly unless mounted to blow horizontally.  Ball bearing fans can be mounted in any orientation but are typically loud. Fluid dynamic bearings are the best in every sense and are worth the extra dollar or two.
 

thanx for the new bios post!!

for me on acx 760 update the bios helped to speed fans down to 31% but the whistle that do turbinnes outdoor games is always this is normal, if noise turbinnes you want it more must go under water cooling.
 

hybrid2d4x4
I know this has been posted a few times before for the 780 cards, but I'm also extremely disappointed in the high minimum allowable fan speed on the ACX 760 card. This thing idles at 23°C, and at 43% fan speed, is probably 15-20dB louder than the rest of my system. This makes this the coolest-running at idle card I've ever encountered, and the tradeoff is that the fan noise is a LOT higher than it needs to be at idle.

If anyone at EVGA reads this, can you please pass on my request to release a BIOS that doesn't have an artificially restricted minimum fan speed? I've read about 780 users flashing to the Gigabyte BIOS and running @ 17% to fix their issue, and I'd like to do the same but I haven't come across any 760 owners that have tried this or that can recommend an equivalent non-EVGA BIOS.

Lastly, as a pre-emptive comment to those that claim that "the ACX cooler is soooo quiet they can't hear it", etc.= If you think it's quiet then I'm happy for you. Odds are you either have lousy hearing or the rest of your system is equally loud, in which case this thread does not concern you. Please do not litter this thread with some sort of misguided defense of EVGA's honour. The facts are plain as can be: the card idles extremely cool @ 23°C and I perceive the fans to be way too loud, so there is a LOT of thermal headroom, and there should be no physical reason for not being to run below 43%.

What case are you using and how many fan's are you using. Also what are the rpm's of the case fans?